A 15 mile section through downtown Phoenix will complete Interstate 10. After years of delay, the project is now close to completion. The project required construction of 6.5 miles of drainage tunnels. The tunnels were selected from some 21 alternate designs to solve the problem of drainage, particularly impacting a 3 mile depressed freeway section. Local rainfall conditions, a high degree of urbanization and the location of the freeway between an extensive portion of watershed and the natural outlet for runoff would create potential for flooding. Three tunnels will convey runoff without disrupting traffic and the downtown economy. The cost of this alternative is half of that of conventional drainage systems. Two of the tunnels with outfalls at the Salt River will act as inverted siphons, thereby overcoming a problem caused by the flat topographic relief of the Phoenix Basin. Runoff is intercepted above the freeway by a conventional near-surface sewer system and conveyed into the tunnels. Runoff falling on the depressed freeway will be pumped to the near-surface conveyance and on into the tunnels. Since the tunnels will be subjected to surge conditions, they are designed as pressure conduits.